S1P (sphingosine-1-phosphate) is produced via an intracellular ceramide pathway, in which ceramide is the starting material. Ceramide is produced via two pathways, the first of which is de novo biosynthetic pathway. Ceramide is also produced by the degradation of sphingomyelin, a cell membrane constituent, in a cell. The S1P level in each tissue is controlled by two biosynthetic sphingosine kinases (SphKs) and two biodegradable S1P phosphatases (S1P lyase and lysophospholipid phosphatases). S1P, produced via phosphorylation of sphingosine by sphingosine kinase, is known to mediate various cellular responses, such as cell proliferation, cytoskeletal organization and migration, adherence- and tight junction assembly, and morphogenesis. S1P exists as a combined form with plasma protein including albumin at high level (100˜1000 nM) in plasma, while it is at a low level in tissues.
S1P binds with S1P receptor, a G-protein coupled receptor, to show various biological functions. As S1P receptor sub-types, S1P1˜S1P5 are known up to now and are named endothelial differentiation gene (EDG) receptors 1, 5, 3, 6 and 8, respectively. The S1P receptors are known to be involved in various biological functions such as leukocyte recirculation, neural cell proliferation, morphological changes, migration, endothelial function, vasoregulation and cardiovascular development.
In recent years, many studies have found that the S1P signaling process via these receptors plays an important role in a series of responses related to multiple sclerosis including inflammation response and the repair process, and a non-selective S1P1 agonist was actually approved as a therapeutic agent for multiple sclerosis. S1P receptors are extensively expressed in many cells related to the induction of multiple sclerosis. Especially, S1P1 receptor plays a major role in the immune system. S1P1 receptor is mainly expressed on the surface of lymphocytes such as T cell and B cell, and responds to S1P resulting in involvement in recirculation of lymphocytes. In normal condition, the S1P concentration is higher in body fluid than in lymphoid tissue, and therefore lymphocytes leave lymphoid tissue by the difference of S1P concentration to circulate after efferent lymph circulates. However, if S1P1 receptor in lymphocytes is down-regulated by S1P1 agonist, the egress of lymphocytes from lymphoid tissue does not occur, resulting in reduced infiltration of autoaggressive lymphocytes which cause inflammation and tissue damage in the central nervous system (CNS). As a result, a therapeutic effect on multiple sclerosis is obtained. Fingolimod, a non-selective S1P1 agonist, has been approved as an oral medication for the treatment of multiple sclerosis. When it binds at S1P1 receptor to be activated, the receptor becomes degraded or internalized from the surface of lymphocytes ironically, and thus it acts as a functional S1P1 antagonism.